uml: Resolve leak if need to requery in umlIdentifyOneChrPTY()

Coverity noted that in the retry logic loop if res had been set, then
it could be leaked so add a VIR_FREE(res) prior to retry.

src/uml/uml_driver.c

 /*
  * uml_driver.c: core driver methods for managing UML guests
  *
- * Copyright (C) 2006-2012 Red Hat, Inc.
+ * Copyright (C) 2006-2013 Red Hat, Inc.
  * Copyright (C) 2006-2008 Daniel P. Berrange
  *
  * This library is free software; you can redistribute it and/or
@@ -261,6 +261,7 @@ requery:
         /* XXX should do this in a better non-blocking
            way somehow ...perhaps register a timer */
         if (retries++ < 50) {
+            VIR_FREE(res);
             usleep(1000*10);
             goto requery;
         }